Analog and digital electronic circuitry and attendant wiring may encounter serious operating difficulty in the presence of strong electromagnetic fields. Such fields are generally referred to as Electromagnetic Interference (EMI) fields. The circuits and attendant wiring may be shielded and filtered to provide some immunity to large EMI fields, however, it is not possible or practical to design the circuitry and attendant wiring so as to ensure immunity to EMI fields. Methods and apparatus, therefore, are required to test the susceptibility of the circuits and attendant wiring to EMI fields.
EMI testing is typically performed in shielded enclosures, known as "screen rooms", which provide an electromagnetic environment wherein only controlled EMI fields are present. Apparatus typically used inside the screen room includes current probes attached to a harness wire and a transmitter which sends the signals detected by the probes to a receiver outside the screen room, where the effects of the EMI fields on the circuit are determined.
To ensure the integrity of the screen room and the results of the EMI tests, any voltage measuring apparatus within the screen room should minimally perturb the controlled EMI fields and should be energized by a signal from the device under test only. For example, any test apparatus which might reradiate EMI fields impinging on the device under test or might otherwise inject any noise into the device under test must be avoided.
Probes suitable for monitoring current during EMI tests are commercially available. The Ailtech model number 91197-11 is one such device. Voltage probes are available, however, none are adequate to monitor voltages during EMI radiated susceptibility testing. Existing wire probes act as an antenna in the screen room, picking up energy from the controlled EMI fields. The induced energy on the probes may be injected into the device under test at the point of monitoring, where it can degrade performance. Existing wire probes, therefore, may cause an erroneous indication of susceptibility to EMI fields.
The lack of suitable wire voltage probes for use in EMI testing has resulted in the promulgation of test procedures, in certain industries, specifying that wired instrumentation, such as voltage probes, not be used.